1. Field of the Invention
The present invention relates to an interferometer for the precise measurement of the configurations of the surfaces of lenses, optical elements, etc., particularly the configurations of spherical and aspherical surfaces.
2. Description of the Prior Art
In general, an interferometer measures the configuration of a test surface using the interference pattern created by the reflected beam from the test surface of a test object and that from the reference surface. Such an interferometer is equipped with a test beam generator which forms a wave front corresponding to the design configuration of the test surface in order to obtain the reflected beam from the test surface. The wave front generated by the test beam generator is a spherical wave where the test surface is spherical, and an aspherical wave where it is aspherical.
Incidentally, innumerable virtual configurations exist for the wave front generated by the test beam generator depending on the distance from the generator. Naturally, the design configuration of the test surface is included in these virtual configurations. On the other hand, an interferometer normally requires that the test surface be positioned where the distortion of the interference pattern is smallest. This means that the test surface is positioned at one of the abovementioned virtual configurations. In other words, the position of the test surface is adjusted such that an interference pattern is formed by the reflected beam from the test surface, and this interference pattern is white (or black) only--a pattern termed `one color`--or such that it comes closest to that condition.
Where the test surface is spherical, the measuring wave front corresponds to innumerable virtual spherical surfaces (including the design spherical surface). Therefore, if the test surface exactly matches the design spherical surface, naturally an interference pattern of one color may be obtained; it must be noted, however, that when the test surface coincides exactly with one of the innumerable virtual spherical surfaces, an interference pattern of one color is obtained as well. Similarly, where the test object has an aspherical surface, when the test surface coincides with one of the innumerable virtual aspherical surfaces, a one-color interference pattern is also obtained.
As a result, the information which can be obtained from the interference pattern amounts to `the discrepancy between the test surface configuration and a certain virtual spherical surface` or `the discrepancy between the test surface configuration and a certain virtual aspherical surface`. In the case of a spherical surface, the distortion of the interference pattern can be discovered separately by using a curvature prototype, a spherical surface measuring device, etc. However, in the case of an aspherical surface, there has been the problem that there are no such appropriate devices, and that the interference pattern obtained includes `a discrepancy between the test surface configuration and a virtual aspherical surface` and `a discrepancy between the design aspherical surface and a virtual aspherical surface`, where `the discrepancy between the test surface and the design aspherical surface` cannot be easily measured.
The present invention tries to obtain `the discrepancy between the test surface configuration and the design configuration` only, by accurately positioning the test object at a predetermined measuring position. A device to accurately position the test object at a predetermined measuring position has long been suggested. For example, already known are devices such as (a) a device which has a position detection member in addition to a configuration measuring member and (b) a device which moves the test object to the measuring position after the vertex of the test surface is positioned at the point where the measuring wave converges.
First, one example of device (a) which is equipped with a position detection member is provided in Japanese Laid-open Patent Publication No. 61-246634. In this example, there is an alignment beam path in addition to the measuring beam path, and when alignment of the test object is performed, the measuring beam path is closed with a shutter so that the light is led into the alignment beam path and the test object is accurately set to the predetermined measuring position by analyzing the position of light converging on the area sensor.
With a device which moves the test object to the measuring position after positioning the vertex of the test surface at the point where the measuring wave converges, as suggested by (b), the vertex of the test surface is first positioned at the point where the measuring light converges, from which point the test object is then moved to the actual measuring position using a high-precision linear guiding mechanism.
However, when positioning the test object using the abovementioned devices, there have been such shortcomings as complex operation, time-consuming adjustment, and numerous device components.
Regarding device (a), which is equipped with a separate position detection member, components for alignment such as a shutter to switch the beam paths and a lens to converge the alignment light to the area sensor are required, and moreover, errors in manufacturing these components must be taken into consideration. As a result, not only do miniaturization and lower costs for the device become unattainable, but accuracy of measurement is also reduced.
Regarding device (b), because the test object must be moved strictly parallel to the optical axis, the degree of parallelization between the linear guiding mechanism and the optical axis and the reliability of the measurement of the position of the test object must be improved. For that purpose, a high-precision guiding mechanism is needed, which not only complicates the device but also increases the cost.